In this application we shall use the term “mobile” with the meaning of a device mounted on a vehicle as a car, airplane, etc.
Mobile devices such as e.g. receivers, transmitters are more and more used in cars, avionics i.e. in objects that are changing their position in time. The communications are normally implemented using wireless communications. Each communication device used in these environments has a physical layer. The physical layer (PHY) of a wireless communication device usually includes an antenna, a transceiver, and a baseband processing unit. The connection between the antenna and the transceiver normally uses a cable that should have good transfer properties for the signals that are either captured by the antenna or have to be sent by the antenna as e.g. small reactance, small resistance. These signals are normally modulated at a Radio Frequency (RF), such that the propagation through the air is possible. The cable properties have to be such that the modulated signals experience hardly any distortion and attenuation, e.g. coaxial cables. Since attenuation scales with the cable length, cable length should be kept to a minimum. Also, for cost reasons, there is a motivation to reduce the use of lengthy cables with stringent transfer characteristics to a minimum.
In some cases, a long cable cannot be easily avoided. For example, a communication device in a car experiences the challenges of having an antenna at a position that is favorable for reception/transmission conditions e.g. on the roof of the car and a transceiver and baseband processing unit that is placed at the head-unit or in the trunk. The trend of using more antennas for Receiving/Transmitting (Rx/Tx) diversity and Multiple Input Multiple Output (MIMO) for improving the communication properties as e.g. robustness, throughput increases the need of lengthy cables.
Moreover, it is desirable to have a scalable solution such that a multi-antenna communication device delivering a service that can be easily extended with more communication devices which deliver a multitude of services without requiring a lot of costly cables.
RF cables can be avoided when the transceiver is located close to the antenna. When the baseband processing unit is placed close to the antenna, no expensive RF cables are necessary.
However, this approach, although possible, severely limits the potential diversity gain of multiple antenna solutions. A solution that combines scalability with antenna diversity is shown in FIG. 1. In this solution, several baseband signals of DVB-T demodulators are combined in order to achieve diversity gain. The system comprises three receivers 100, 105 and 110, each receiver being connected to a respective antenna 103, 108, 113 for receiving RF signals. Each receiver comprises a respective tuner 101, 106, 111 and a respective baseband i.e. DVB-T demodulators 102, 107, 112 for demodulating the signals received from the respective tuners. A drawback of this solution is that the DVB-T demodulators have to be co-located close to each other and therefore lengthy costly cables cannot be avoided between the antennas and tuners.
An alternative solution is shown in FIG. 2. FIG. 2 depicts the structure of a signal used on a serial interface targeting the communications among multiple tuner chips and a baseband chip.
The signal comprises a header, the header including a whitening seed W and synchronizing bits Y. The header is followed by a payload comprising the IQ signal and the status bits. However, the system of FIG. 2 shares the same oscillator, and this is not possible in a distributed architecture involving long distances between its multiple components. The information shared among the multiple tuner ICs includes a forwarded clock signal. The system provides a clock recovery from a whitened data signal included in the transmitted data.
Part of the Automated Gain Control (AGC) loop of this solution is done over the communication channel. This might be an unnecessary limitation as the AGC loop can be implemented close to the antenna but also close to the communication channel.
The structure of a frame for exchanging data between tuner and baseband processing is shown in FIG. 2 and this specific structure can be further used in other configurations.
A tuner mat comprise typically an input circuitry for adapting to an antenna, the antenna receiving an incoming RF radio signal in a specific frequency range and having a certain modulation as e.g. frequency modulation (FM), amplitude modulation (AM), orthogonal frequency division multiplexing (OFDM), etc. The tuner further comprises an amplifier for amplifying the incoming RF signal and for delivering an amplified RF signal. The tuner further comprises a local oscillator (LO) which generates a signal of frequency fo which is provided to a mixer. The mixer further receives the amplified RF signal having a frequency fRF and delivers an intermediate frequency (IF) signal which has the frequency fI=fRF−fo. If fRF=fo, then the resulting signal is situated in the baseband and the receiver is called zero-IF receiver. The mixer may be active or passive. For quadrature modulated input signals as the OFDM ones, the mixer comprises two mixing sections: one for the in-phase component (I) and another one for Quadrature one (Q). Usually, a feedback from the mixer to the amplifier is provided in order to keep the overall gain of the amplifier at a desired level, independent of the input frequency. This feedback is usually defined as the Automatic Gain Control (AGC). The feedback is usually controlled by a controller which is adapting it at appropriate time events as e.g. during the preambles of the packets or during guard intervals. The control data from the controller, which normally is a digital one, might be transmitted to the next stages in the radio. The radio may also have a Received Signal Strength Indication (RSSI) which is an indication of the received RF signal intensity. The RSSI signal is also available as a digital signal and therefore it can be easily transmitted to other stages. The AGC and RSSI can be easily embedded in any serially transmitted information provided that a parallel to serial converter is provided.
The Analog to digital converter (ADC) transforms the IF signal into a digital one. The ADC acquisition process and output generation is controlled by a clock signal (Ck). Without limitation, the ADC could be either a Nyquist e.g. flash, Successive Approximation (SAR) one or Sigma-Delta one (SD). The digital signal may be present as a parallel digital word for example as in the flash AD converter case. In this case serializer i.e. a parallel to serial converter, is connected to the output of the converter that transforms the parallel digital word into a serial one. Alternatively, the ADC may provide directly a serial output.
A baseband system is considered to receive the digital IF and to demodulate it according to the type of modulation of the signal. This demodulation applies specific algorithms that might be implemented either in hardware, software or a combination thereof. The demodulation could be made in full i.e. at the output of the baseband system the demodulated useful signal is obtained, or partially i.e. part of the demodulation is carried out in a first subsystem and the remaining part is carried out in a different subsystem, not necessary situated on the same chip or module with the first subsystem.
In wireless communications, diversity gain is considered to be the increase in signal-to-noise ratio due to some diversity scheme, or how much the transmission power can be reduced when a diversity scheme is introduced, without a performance loss. Diversity gain is usually expressed in decibel, and sometimes as a power ratio.
Throughout the present description, equal reference numbers used in different figures refer to same devices or features.